This invention relates to the field of Integrated Circuit (IC) fabrication and assembly, and in particular to the fabrication of an encapsulated multi-chip device, such as a multi-die LED with an integral encapsulation lens.
As the light emitting capabilities of solid state light emitting devices1 (LEDs) continues to improve, their use in conventional lighting applications continues to increase, as does the competitive pressures to provide reliable, long-lasting products in a cost-effective manner. Even though the cost of LED products is relatively low, the savings of even a few cents per device can have a significant impact on profit margin, due to the increasingly growing market for these devices. 1 For the purposes of this disclosure, the acronym ‘LED’ refers to a light emitting device; a light emitting diode being an example of such a light emitting device.
In the field of high-flux/lumen devices, multiple LEDs may be encapsulated in a single optical encapsulation material, such as a conventional dome-shaped epoxy lens. A common arrangement for producing white light is a combination of one or more yellow LEDs and one or more red LEDs. The yellow LEDs may be formed using InGaN as the emitting material, and the red LEDs may be formed using AlInGaP as the emitting material. The multiple LEDs are typically arranged on a common substrate, using, for example, Device on Ceramic (DoC) techniques, wherein the common substrate is a ceramic, such as aluminum nitrate (AlN) or alumina (Al2O3).
FIGS. 1A (top view) and 1B (side view) illustrate an example LED element 100 with a multi-chip LED combination 110 and a domed encapsulation 120. In this example, the LED combination 110 includes two yellow LED chips 112 (which may be formed, for example, by blue light emitters and phosphor wavelength converters) and six red LED chips 114. In order to provide appropriate optical characteristics for efficient light-extraction, the encapsulation 120 occupies a substantially larger surface area than the LED combination 110, and a correspondingly large substrate 130 may be needed to accommodate this encapsulation area.
The substrate 130 comprises a material that is sufficient to accommodate the processes involved in forming the LED dies, and preferably has appropriate thermal propagation characteristics to transfer the heat produced by the dies, thereby lengthening the operational life of the LED. As noted above, ceramic material, such as aluminum nitrate (AlN) or alumina (Al2O3), is commonly used, although other substances may also be used.
In a typical embodiment, the substrate material 130 may amount to at least 8-10 times the area of the LED arrangement 110. Additional area may also be required to provide the interconnections required to connect the structure 100 to the appropriate circuitry required to operate the LED array 110.
The conventional LED element 100 may be fabricated by forming a plurality of LED arrangements 110 on a single ceramic tile 150. For example, as illustrated in FIG. 1C, a typical 100 mm×100 mm ceramic tile may be used as a common substrate to form a 10×10 array of substrate elements 130 to provide a hundred LED elements 100. After fabrication, the 100 mm×100 mm ceramic tile may be sliced/diced to provide these hundred LED elements 100.
To minimize fabrication costs in the conventional processes, the encapsulation of the LED elements 100 may be performed while the structures are contained on the un-diced ceramic tile, so that all of the hundred LED elements 100 can be encapsulated in a single encapsulation process, using, for example, a single mold with corresponding 10×10 domed shapes. In like manner, if other manufacturing steps are required, such as the addition of phosphorescent material that provides light of a different wavelength than the wavelength(s) of the light generated by the LED combination 110, these steps may be performed while the LED elements 100 are on the un-diced ceramic tile.
The use of a larger ceramic tile would generally be preferred, to allow each processing step to be applied to more LED elements 100, thereby reducing manufacturing costs. For example, a 150 mm×150 mm tile will provide for the production of more than twice as many structures 100 than the conventional 100 mm×100 mm tile, while using the same processing steps with minimal additional costs. However, manufacturing limitations and quality control limitations do not currently allow for the use of tiles much larger than 100 mm×100 mm.
Because each of the aforementioned production steps may be performed with the fabricated LED arrays on the un-diced ceramic tile, both operative and inoperative LED arrangements 110 are fully processed, such that the cost of encapsulation and optional phosphorescent coating is incurred for each of the LED elements 100 on the tile 150, regardless of whether the encapsulated LED element 100 is operative or not.
To reduce manufacturing costs, the individual LED elements 100 on the tile 150 may be electrically connected to each other, to provide, for example, a single connection point for applying a common bias to facilitate etching, a common voltage to facilitate testing, and so on. When the tile is diced to create the finished LED elements 100, these connections between the elements 100 will be exposed on the edges of each element 100. Consumer safety groups, such as Underwriters Laboratory (UL) in the United States, may prohibit such exposed electrical connections when the LED element 100 is used in high voltage (110 v) applications, such as light bulbs. Even though the LED element 100 may be enclosed within a glass bulb, a possible breaking of the bulb should not allow any high voltage connections to be exposed. Because of this requirement, an additional protective layer may be applied to or over each LED element 100 after it is diced from the tile 150, to insulate any exposed connections, thereby increasing the manufacturing costs for such LED elements 100.